
Man something feels suspicious about this. What was that leaf, what exactly did it do, why did Henrick put it in his own mouth first, what's up with the sudden fever, what's up with that sudden kiss?
Suspicious, me thinks

No need to overthink lol Heine's saliva has healing properties, same with the rest of the other people on his team that are injected with the x virus. The reason why he used a leaf is because if he directly used his lips, it tends to make the other person lose their mind as they experience extreme pleasure. There's an explanation in the novel for that. Basically, their body, including their saliva is powerful, that's why they rarely have sex with normal people also because they're too strong for that.
In simpler way, if they have sex, they could break the other person's bones etc but their saliva can heal them and give them extreme pleasure at the same time.
Example would be Heine's first time with Xiao Yan lol poor guy didn't stand a chance.
Fucking knew her ass was bothersome, nosy ass bitch.